Cathode-grid assembly with shielding means to prevent deposition of conductive material on insulating support



Oct. 3, 1967 3,345,527

E. ATTI CATHODE-GRID ASSEMBLY WITH SHIELDING MEANS 'TO PREVENT DEPOSITION OF' CONDUCTIVE MATERIAL ON INSULATING SUPPORT Filed June 24, 1965 WW Eros Atti @me www@ ATTORNEY Unted States Patent O CATHODE-GRID ASSEMBLY WITH SHIELDING MEANS T PREVENT DEPOSITION GF CGN- DUCTIVE MATERIAL ON lNSULATING SUP- PGRT Eros Atti, Horseheads, N.Y., assigner to Westinghouse Electric Corporation, Pittsburgh, Pa., a corporation of Pennsylvania Filed June 24, 1965, Ser. No. 466,769 7 Claims. (Cl. 313-82) This invention relates to improved electron discharge devices `and more particularly to cathode-grid assemblies which may be incorporated in such devices.

More specifically, this invention relates to a cathode grid assembly which may be incorporated into a cathode ray tube. Typically, a cathode ray tube provides a plurality of electrodes within the envelope of the tube 'for forming the electrons emitted by the cathode-grid assembly into a beam and for directing the electron beam onto a iluorescent screen to provide a visible light image. The Icathode-grid assembly normally includes a cathode element with a metallic sleeve having one end closed by a wall portion upon which there has been deposited a thermionic electron emitting material. Electrons are emitted from the aforementioned material when the metallic sleeve is heated to an elevated temperature. The cathode element is heated by a filament which is positioned within the metallic sleeve of the cathode element. During operation of this device, an electric current is passed through the heater lament to thereby maintain the heater at incandescence and to keep the cathode coating at a temper- -ature adequate for electron emission. Further, the sleeve of the cathode element is supported within a cup-shaped control grid having an enclosed end wall with a centrally orientated aperture therein. Typically, a ydisc having an aperture therein and being made of a suitable electrically insulating material such as aluminum oxide supports the cathode sleeve axially of the control grid and at a predetermined distance from the end wall of the control grid. A prime function of the cathode support disc is to electrically insulate the cathode element from the control grid element, which elements are normally maintained at different potentials.

During the operation of the cathode ray tube, the cathode sleeve and the coating of electron emissive material are raised to extreme temperatures. At these high temperatures, vapors are emitted from the electron emissive coating and from the metallic cathode sleeve itself; these materials tend to deposit on those surrounding, cooler surfaces such as the insulating support dis-c in the form of electrically conductive lms. As the lilms of the conductive material continue to be deposited and increase in thickness, the electrical insulation between the control grid and the cathode element tends to break down.

Cathode sleeves may be made of such suitable electrically conductive materials such as nickel or molybdenum. For the example of nickel, it is noted that the evaporation rate of nickel increases approximately 10 fold when the temperature is raised from 830 C. to 890 C. and 10() fold for a change of temperature from 830 C. to 980 C. Thus, it may be seen that the evaporation of metallic substances from the cathode sleeve may play an important role in breaking down the electrical insulation between the control grid and the cathode element.

As a result of the grid to Icathode leakage, the characteristics of the electron gun, of which the grid-cathode assembly are a part, may be severely impaired or even substantially destroyed. Primarily, leakage is objectional because it tends to cause a change in the potential difference applied between the cathode element and the control grid thus effecting the brightness or contrast setting. Large 3,345,527 Patented Oct. 3, 1967 ICC leakages may even result in the complete loss of intensity control of the electron beam.

In the past, cathode grid assemblies have been assembled with insulating support discs having a groove therein. The grooves are formed on both sides of the insulating support disk and in a continuous circle about the center aperture in which the cathode sleeve is disposed. Thus by providing the discontinuity in the surface of the insulating support disc, the deposition of the conductive materials across the insulator surface may be interrupted an-d the electrical insulation between the cathode and grid elements maintained. More specically, the angles of incidence upon the insulating disc of the conductive materials emitted by the cathode element are such that the wall of the groove adjacent the cathode sleeve and a part of the bottom of the groove are shielded from suc-h materials.

However, as electron guns are made smaller, the insulating support discs are correspondingly made smaller so that the depth of the grooves placed on either side of the support discs and their actual distance from the central aperture of the insulating disk are quite small. It has been found impracticable to displace :these grooves sufficiently from the central aperture to provide the required shielding eifect necessary to maintain insulation between the cathode element and control grid.

Further, the prior art also teaches .the use of shields to be placed within the grooves to provide a more effective shielding between the walls of the groove and the cathode element. Thus, a further portion of the groove is protected or shielded from the presence of a conducting substance. However, the shield so disposed within the grooves does not provide shielding for the entire surface of the groove; for example, the conductive materials emitted from the electron emissive coating and from those portions of the cathode sleeve, remote from the insulating disk are still allowed to directly deposit upon portions of the groove. Further, it may be understood that due to particle rellections the sublimation or evaporation of the conductive materials does not necessarily proceed in a direct line and that during extended periods of continued use of these cathode grid assemblies, the conductive films will deposit even in those areas that are so shielded.

It is therefore an object of the present invention to provide an improved electron discharge device.

It is a further object of this invention to provide a new and improved cathode-grid assembly for an electron ldischarge device in which the cathode to grid insulation is maintained over extended periods of operation.

It is another object of this invention to provide a new and improved cathode-grid assembly for an electron discharge device wherein an electrically conductive `coating path is prevented from forming between the cathode element and the control grid.

A still further object of this invention is to provide a new and improved cathode-grid assembly for .an electron discharge `device wherein substantially the entire area of the discontinuities provided by the insulating disc is completely shielded from the sources of conductive films.

It is another object of the invention to provide a new and improved cathode-grid assembly for an electron discharge device wherein means for shielding the discontinuities provided by the insulating disc may be easily incorporated into existing cathode-grid assemblies.

Briefly, the present invention accomplishes the above cited objects by providing a lcathode-grid assembly for an electron discharge device in which the insulating support disc upon which the cathode sleeve is mounted has a continuous groove therein which is substantially shielded from the direct sublimation of or evaporation of conductive materials. More specifically, a shielding means is inserted according to the teachings of this invention within the cathode-grid assembly so that the conductive materials thrown off by the cathode element are either intercepted by said shielding means or strike the surface of the insulating disk adjacent the groove at small, grazing angles.

Further objects and advantages of the invention will become apparent as the following description proceeds and features of novelty which characterize the invention will be pointed out in particularity in the claims annexed to and forming a part of this specification.

For a better understanding of the invention, reference may be had to the accompanying drawings, in which:

FIGURE 1 illustrates an electron discharge device embodying the teachings of this invention; and

FIGS. 2, 3, 4, 5 and 6 present partially sectioned views of differing embodiments of the cathode-grid assemblies which may be incorporated into the electron discharge device -of FIGURE 1.

Referring now to the drawings and in particular to FIG. 1, there is shown a cathode ray device as a I specific embodiment of this invention. The cathode ray device 10 comprises an evacuated envelope 11 of a suitable material such as glass with an elongated portion 12 and a flared portion 13. An electron gun 14 is mounted axially within the elongated portion 12 so that a beam of electrons emitted from the electron gun 14 will strike a display target 17 coated upon the enclosed end of the envelope 11. The elongated portion 12 of the envelope 11 is enclosed by a base 19 through which a plurality of terminals 18 are mounted for providing electrical signals to the various elements within the envelope 11. The electron gun 14 includes accelerating and focusing elements 16 and 15 respectively, and a cathode-grid assembly 20. A deflection means (not shown) of either the electromagnetic or electrostatic variety may be mounted outside or inside the envelope so as to control the deflection of the electron beam emitted from the electron gun 14.

Referring now to FIG. 2, an illustrative embodiment of the cathode-grid assembly 20 is shown in detail. The cathode-grid assembly 20 comprises a cathode sub-assembly 22 including a cathode sleeve 24 made of a suitable electrically conductive material such as nickel and enclosed at one end by a cap 26. The outer surface of the end cap 26 is coated with a layer 28 of a suitable thermionic electron emissive material such as a mixture of strontium and barium carbonates. Disposed within the cathode sub-assembly 22 is a heater element 30 which is coated with a layer of electrically insulating material. Further, a terminal lead 29 is connected to the cathode sleeve 24 as by spot welding. The cathode sub-assembly 22 is supported within a central aperture 34 of an insulating support member or disc 32 made of suitable electrically insulating materials such as steatite, or aluminum oxide by peripheral bulges 36 and 38 which are pressed out of the wall of the cathode sleeve 24 into contact with the opposite surfaces of the support disc 32. The support disc 32 and the cathode sub-assembly 22 are in turn supported within a control grid cup 40. The control grid cup 40 includes a metallic cylinder 42 enclosed upon one end by an annular flange 44 which forms at its center a grid aperture 46. The grid aperture 46 is so aligned with respect to the cathode sub-assembly 22 and the electron emissive layer 28 to allow the electrons emitted from the layer 28 to pass therethrough.

The distance between the electron emissive layer 28 and the inner surface of the annular flange 44 of the control grid cup 40 is maintained at a critical small distance of only a few thousandths of an inch by the insertion of a spacer element 50 between the insulating support disc 32 and the annular flange 44. In order to lock the cathode sub-assembly 22 and the insulating support disc 32 within the control grid cup 40, an annular disc 58 having an integrally formed flange 64 is pressed inwardly against the adjacent surface of the support `disc 32 and then is secured to the inner surface of the control grid cup 40 by a suitable method such as spot welding.

During the operation of the cathode ray device 10, a potential difference is applied across a pair of terminals 31 of the heater element 30. The `current flow through the heater element 30 generates suicient heat to raise the temperature of the cathode sleeve 24 and the end cap 26 to that temperature at which the electron emissive layer 28 will generate a substantial electron flow. In addition, to emitting a ow of electrons, the electron emissive layer 23 will also throw off particles of electrically conductive substances which tend to deposit on the surrounding, cooler elements. In matrix type of cathode elements, metallic substances such as nickel are embedded within the element and will be thrown off at the high operating temperatures. Further, these conductive substances tend to deposit upon and form a thin conductive lm across the insulating support disc 32 between the cathode subassembly 22 and the control grid cup 40.

Normally, the cathode sub-assembly 22 is typically maintained at a negative potential difference with respect to the control grid cup 40 within a range of between -70 volts at beam cutoff and 0 volt at full beam to insure the proper control of the electron beam emitted by the layer 28. However, if a conductive connection is established between the cathode sub-assembly 22 and the control grid cup 40 the desired voltage and thus the operation of the cathode ray device 10 may not be maintained. Further, due to the fact that the cathode sleeve 24 of the cathode sub-assembly 22 is being operated at a high temperature and in an evacuated environment, metal from the outer surface of the cathode sleeve 24 will also be evaporated or emitted and Will also deposit on the cooler surfaces of adjacent electrodes. Thus, the metallic, electrically conductive substances thrown oif by the cathode sleeve 24 will, as does the conductive materials thrown off by the electron emissive layer 28, 4deposit uipon the insulating support disc 32 to `form a conductive film between the cathode sub-assembly 22 and the control grid cup 40.

In order to prevent the formation of an electrical connection between the cathode sub-assembly 22 and the control grid cup 40, a pair of grooves or discontinuities 35 and 37 are formed on both surfaces of the insulating support disc 32. The grooves 35 and 37 serve to break up the electrical connection formed 'by the deposition of the conductive film from both the cathode sleeve 24 and the electron emissive layer 28 by forming areas which are substantially shielded from the above-mentioned sources of conductive materia-1S. More specifically, the grooves 35 and 37 respectively have walls 39 and 41 which are hidden from and substantially shield the remaining areas within the grooves 35 and 37 from the direct impact of the conductive particles ejected from the hot cathode element. However, during the extended and/ or intensive use of the cathode grid assembly 20, the conductive ymaterials will tend to coat even the Walls 39 and 41 as well as the shadowed areas of the grooves 35 and 37 to thereby significantly lower the electrical insulation between the cathode sub-assembly 22 and the control grid cup 40.

The deposition of conductive materials from the electron emissive layer and the cathode sleeve may be substantially reduced by inserting a shielding means so that the conductive materials generated by the electron emissive layer may not directly strike the grooves within the insulating support discs and that the angle of incidence of these conductive materials upon the surface of the insulating disc 32 is greatly reduced so that a significantly larger area beneath the interior walls 39 and 41 of the grooves 35 and 37 is shielded from the conductive materials. Referring to FIG. 2, the shielding means is provided Iby the spacer element 50 which includes a at portion 52 which directly abuts against the portion of the insulating support disc 32 between the groove 35 and the inner wall of the cylinder 42. An extended portion 54 of the spacer element 50 provides the spacing means between the insulating support disc 32 and the inner surface of the .annular flange 44 of the control g-rid cup 40. In addition, a shielding portion 56, formed integrally with the spacer element 50, is so positioned with respect to the electron emissive layer 28 and the cathode sleeve 24 that the trajectory (line A) of the conductive material particles ejected by the layer 28 does not fall within the groove 35 and that the trajectory (line B) of the conductive material evaporated from the cathode sleeve 24 makes a small, grazing angle C with respect to the surface of the disk 32 adjacent groove 35. Thus, the shielding means in accordance with the invention provides a significantly increased area within the groove that is shielded from the direct impact of electron emissive materials.

Further, the annular disc 58 is so disposed on the other surface of the insulating support disc 32 to sub stantially reduce the angle of incidence With respect to the sur-face of the disk 32 of the conductive particles emitted from the surface of lthe cathode sleeve 24. Further, the annular disc 58 comprises an annular abutting portion 60 which is disposed against that portion of the surface of the insulating supporting disc 32 between the groove 37 and the interior wall of the metallic cylinder 42. In addition, an integrally formed shielding portion 62 is disposed in a spaced (so as not to make electrical contact) relation with the sur-face of the insulating support disc 32 so that the trajectories (line D) of the particles ejected from the cathode sleeve 24 strike the surface of the insulating support disc 32 at a relatively small angle E. It is desirable that the conductive materials thrown olf by the cathode sleeve 24 strike the surface of the insulating disc 32 near the groove 35 at a grazing angle close to being parallel with the surface of the disc 32. In any case, the trajectory of the conductive materials should not strike the surface of the disc at an angle greater than 25 Y FIGURE 3 shows yan alternative embodiment of the invention shown in FIG. 2 in which the same reference numerals are used for identical elements. In the embodiment as shown in FIG. 3, a modified spacer element 70 is disposed between the insulating support disc 32 and the inner surface of the annular flange 44. In particular, the spacer element 70 includes a flat portion 72 which abuts against the surface of the support disc 32 and an extended portion 74 providing the spacing between the aforementioned surfaces. The shielding means is provided by a cup shaped shield 76 which is spaced from the surface of the support disc 32 and is secured las by spot welding wit-h the extended portion 74 of the spacer element 70 by an integrally formed flange 78. Further, the shield 76 is so placed with respect to the cathode sleeve 24 and the support disk 32, that the trajectories of the conductive particles ejected from the electron emissive layer 28 are substantially prevented from striking the groove 35 and that the trajectories of the particles thrown off by the cathode strike the surface of the insulating support disc 32 at relatively small or grazing angles.

t A further modification of the cathode-grid assembly 20 of this invention is shown in FIG. 4. In the modification of FIG. 4, the spacer element 70 is substantially as that shown within FIG. 3. The shielding means is provided by a first annular shielding disc 80 which has been inserted between the surface of the insulating support disc 32 and the flat portion 72 of the spacer element 70. The first shielding disc 80 includes an abutting portion 82 which is disposed against the insulating disc 32 and between the groo-ve 35 and the inner surface of the cylinder 42, and a shielding portion r84 which is spaced from the insulating support disc 32. The shielding portion 84 extends radially toward the cathode sleeve 24 so that the trajectories of the particles of the conductive material emitted from the emissive layer 28 do not strike the groove 35 and that the trajectories of the particles of the conductive material emitted by the cathode sleeve 24 strike the sur-face of the insulating disc 32 at only a relatively small o-r grazing angle. Further, a second shielding disc 86 is disposed between an annular flange 92 which is securedly attached to the inner surface of the cylinder 42 and the surface of the insulating disc 32. The second shielding disc 86 comprises an abutting portion 8S disposed between the annular flange 92 and the insulating disc 32 and a shielding portion 90 which is spaced from the insulating disc 32 and which extends radially toward the cathode sleeve 24. The second shielding disc 86 is so disposed with respect to the insulating disc 32 and the cathode sleeve 24 that the trajectories of the particles of the conductive material emitted `by the cathode sleeve 24 strike the surface of the insulating disk 32 at a relatively small angles. Further, it is a significant aspect of the embodiment shown in FIG. 4 that the shielding disks be made of a compliant, flexible material such as a sheet of stainless steel with a .002 inch thickness. Thus, by providing a compliant disk `80, the spacing between the surface of the insulating disk and the flange 44 may be accurately determined without introducing errors due to the warpage or bending of the disk 80.

In a modification of the cathode-grid assembly shown in FIG. 2, FIG. 5 shows a cathode-grid assembly 20 including a spacing element 94 having a fiat portion `96, an extending portion 98 and a shielding portion 100 as explained above. In addition, an annular tip 102 is integrally formed with respect to the shielding portion 100 of the spacing element 94. The annular tip 102 extends within the groove 35 and thus further prevents the deposition of conductive materials thrown off from either the emissive layer 28 or the cathode sleeve 24. On the other side of the insulating support disc 32 from the spacing element 94, there is disposed a shielding disc 104 having an abutting portion 106 which is disposed against a surface of the insulating `disc 32 and a shielding portion which is spaced from the insulating support disc 32 and extends radially toward the cathode sleeve 24. Further, the shielding disc 104 has an irrtegrally formed flange 112 which is secured as by spot welding to the interior surface ofthe metallic cylinder 42. In addition to the shielding means provided by the shielding portion 110, a circular ridge 108 is formed in the shielding disc 104 to further prevent the deposition of `a metallic film within the groove 37.

In a further embodiment of the cathode-grid assembly of this invention, there is shown in FIG. 6 a cathode-grid assembly 20 including the spacer element 70, the shielding discs 80 and 86, and the insulating discI 32 as shown and described with regard to FIGURE 4. The significant feature of this embodiment is the provision of circular ridges or Iprojections 114 and 116 upon opposite surfaces of the insulating disc 32. The ridges 114 and 116 are disposed beneath and spaced from the shielding discs 80 and 86 respectively so that the particles of conductive materials emitted from the cathode sub-assembly 22 do not deposit directly upon those portions of the surface of the disc 32 which are shadowed by the ridges 114 and 116 or are reflected in substantial numbers upon these aforementioned portions. It is noted that a plurality of these ridges could be disposed on the surface of the disc 32 or that the ridges 114 and 116` could be used in conjunction with grooves 118 and 120 respectively to further prevent the deposition of a conductive material upon the surfaces of the insulating disc 32.

Thus there has been shown a cathode grid assembly for electron discharge devices such as a cathode ray tube which substantially prevents the deposition of a coriductive layer between the cathode sub-:assembly and the control grid c up. Thus, in accordance with this invention, a cathode-grid assembly has been shown which is inexpensive to manufacture and assemble, and in addition substantially reduces the deposition of the electrically conductive material upon the insulating support disc.

While there has been shown and described what are at present considered to be the preferred embodiments of the invention, modifications thereto will readily occur to those skilled in the art. It is not desired therefore, that the invention be limited to the specific arrangements shown and described and it is intended to cover in the appended claims all such modifications as fall within the true spirit and scope of the invention.

I claim as my invention:

1. An electrode assembly for an electron discharge device comprising a first and second electrode, said first electrode including a 4support member and a source of electrons, said source and said member each having a surface of a material having the property of throwing ofi conductive materials at high temperatures, an insulation member for spacing said first and second electrodes apart, a discontinuity formed in at least one surface of said insulation member between said first and second electrodes, and shielding means disposed between said first electrode and said insulation member for substantially enclosing said discontinuity, the trajectories of the conductive material emitted by said source being intercepted by said shielding means and the trajectories of the conductive materials emitted by said member toward said discontinuity intersecting the surface of said insulation member only at small grazing angles.

2. An electrode assembly for an electron discharge device comprising first and second coaxially disposed tubular electrodes, said first electrode having a surface made of a material having the -property of throwing off conductive materials at high temperatures, and having associated therewith a body of electron emissive material, yan annular, insulating member fixed between and spacing said first and second electrodes, said annular member having on at least one surface thereof a continuous groove separating said first and second electrodes, said groove having a defined wall adjacent said first electrode, and `shielding means fixed to one of said first and second electrodes and disposed between said first electrode and said insulating member for enclosing said groove so that the lines defined by said body of electron emissive material and said groove are intercepted by said shielding means and that the lines defined by a point on said surface of electrically conductive material and by said wall intersect the surface of said insulation member at a small grazing angle.

3. A cathode-grid assembly for a cathode ray tube type device comprising first and second coaxially disposed tubular electrodes with said second electrode enclosing said first electrode, said first electrode having a surface of an electrically conductive material capable of being emitted at high temperatures and having associated therewith a body of electron emissive material, an annular insulation member disposed about said first electrode and spacing said first and second electrodes apart, a continuous groove formed in at least one surface of said insulation member and separating said first and second electrodes, silad groove having a defined wall adjacent said first electrode, said second electrode having an enclosed annular end forming an aperture through which electrons emitted from said body are directed, and a spacing member disposed between said insulation member and said enclosed end of said second electrode, said spacer element having associated therewith a shielding portion disposed between said first electrode and said groove so that the lines defined by said body and said groove are intercepted by said shielding portion and that the lines defined by a point on said surface of electrically conductive material and said wall intersect the surface of said insulation Vmember at small angles less than 25.

4. A cathode-grid assembly for a cathode ray type device substantially as claimed in claim 3, said spacing member having an extended portion thereof disposed within and spaced from the surfaces of said groove.

S. A cathode-grid assembly for a cathode ray type tube, said assembly comprising first and second coaxially disposed tubular electrodes with said first tubular electrode enclosing said second electrode, said first electrode having a first surface made of a material having the property of throwing off conductive materials at high temperatures and having associated therewith ya body of electron emissive material, said second electrode having an enclosed, annular end wall forming an aperture through which electrons emitted by said body are directed, an annular insulation member having an aperture through which said first electrode is disposed for spacing said first and second electrodes apart, a groove formed in at least one surface of said annular insulation member, said groove having a defined wall near said first electrode, a spacer member disposed between said end wall of said second electrode and said `spacing member, and a compliant annular disc disposed between and abutting said spacing member and said insulation member, said annular disc having a portion thereof spaced from said insulation member and disposed between said first electrode and said groove so that the lines defined by said body of electron emissive material and said groove are intercepted by said portion and that the unobstructed lines defined by a point on said first surface and said wall intersect the surface of said insulation member at small grazing angles.

6. An electrode assembly for an electron discharge device comprising first means for emitting a flow of electrons, said first means having the property of throwing off electrically conductive materials, second means for controlling said flow of electrons, third means for supporting said first means, said third means having a surface capable of giving ofi an electrically conductive material, fourth means for supporting said second and third means in an insulated relation, said fourth means having a discontinuity therein to impede the deposition of said electrically conductive materials thereacross, and shielding means disposed between said first means and said discontinuity so that the lines defined by said first means and said discontinuity are intercepted by said shielding means and that the lines defined by a point on the surface of said third means and said discontinuity of said fourth means form small, grazing angles with the surface of said fourth means.

7. An electrode assembly for an electron discharge device comprising a first electrode for emitting a beam of electrons, a second electrode for controlling said beam of electrons, said first electrode having a surface of a material exhibiting the -property of ejecting particles of an electrically conductive material, an insulation member for spacing said first and second electrodes, a projection disposed on the surface of said insulation member and between said first `and second electrodes, and shielding means disposed between said first electrode and said projection and so spaced from said projection that the trajectories of a substantial portion of said particles are intercepted by said shielding means to provide an area protected by said projection from the deposition of said particles.

References Cited UNITED STATES PATENTS 2,641,727 6/1953 Pohle 313-250 2,919,380 12/1959 Barnett 313-82 2,967,963 1/1961 Ballard et al 313-82 JAMES W. LAWRENCE, Primary Examiner.

V. LAFRANCHI, Assistant Examiner. 

1. AN ELECTRODE ASSEMBLY FOR AN ELECTRON DISCHARGE DEVIDE COMPRISING A FIRST AND SECOND ELECTRODE, SAID FIRST ELECTRODE INCLUDING A SUPPORT MEMBER AND A SOURCE OF ELECTRONS, SAID SOURCE AND SAID MEMBER EACH HAVING A SURFACE OF A MATERIAL HAVING THE PROPERTY OF THROWING OFF CONDUCTIVE MATERIALS AT HIGH TEMPERATURES, AN INSULATION MEMBER FOR SPACING SAID FIRST AND SECOND ELECTRODES APART, A DISCONTINUITY FORMED IN AT LEAST ONE SURFACE OF SAID INSULATION MEMBER BETWEEN SAID FIRST AND SECOND ELECTRODES, AND SHIELDING MEANS DISPOSED BETWEEN SAID FIRST ELECTRODE AND SAID INSULATION MEMBER FOR SUBSTANTIALLY ENCLOSING SAID DISCONTINUITY, THE TRAJECTORIES OF THE CONDUCTIVE MATERIAL EMITTED BY SAID SOURCE BEING INTERCEPTED BY SAID SHIELDING MEANS AND THE TRAJECTORIES OF THE CON- 